1. Field of the Invention
This invention relates to high strength, high modulus filaments.
2. Description of the Prior Art
Composite materials have been fabricated which have outstanding strength to density ratios. Typically, these composite materials comprise a high strength, high modulus filament in a plastic or metal matrix. The most widely accepted filaments to date are formed from boron.
Boron filaments are currently manufactured by a chemical vapor deposition process. A small diameter tungsten wire is passed through a reactor containing a gaseous mixture of boron trichloride and hydrogen. To provide the reaction temperatures required, the tungsten wire is resistively heated to high temperatures, typically about 1000.degree. to 1300.degree. C. At these temperatures, boron trichloride is reduced by hydrogen gas and deposits boron onto the tungsten filament to produce a continuous monofilament which exits from the reactor into the ambient atmosphere where it is wound onto spools. Usually, both ends of the reactor are sealed with mercury containing capillaries.
Boron filaments produced by the above method typically have average strengths of about 450,000 p.s.i., an elastic modulus of 60,000,000 p.s.i. and a density of 0.095 pounds per cubic inch. While these materials are relatively strong compared to other available materials, they have only a fraction of the theoretical strength obtainable with the boron. For example, the flexural strength of these filaments is typically about 2,000,000 p.s.i., which suggests that a theoretical tensile strength ought to be achievable of approximately 1.4 million p.s.i. for boron filaments. This has been corroborated by researchers who have measured actual tensile strength in excess of 1,000,000 p.s.i. for coreless boron obtained by longitudinally splitting commercial filaments and chemically etching the tungsten boride core. See Wawner and Satterfield, Appl. Phys. Lett., 11, No. 6, p. 192 (Sept. 15, 1967).
Despite many years of extensive research to raise the strength of boron fibers to values more closely approaching their theoretical limits, a significant gap still exists. Weak areas caused by either low temperature boron deposition or internal stress buildup are still prevalent. Thus, there is a need for boron filament fabrication apparatus and techniques which would eliminate these problems and produce higher strength filaments.